chore(channels): drop admin-side available channels view

Remove the admin-side "Available Channels" aggregate view — admins
already see full channel configuration (groups, pricing, model
mappings) in the channel edit dialog, making a read-only admin
aggregate view redundant. The user-side "可用渠道" remains.

Backend:
- Delete handler/admin/available_channel_handler.go (+ test)
- Drop AdminHandlers.AvailableChannel field and wire injection
- Remove /admin/channels/available route

Frontend:
- Delete views/admin/AvailableChannelsView.vue
- Drop /admin/available-channels router entry
- Strip AvailableChannel types + listAvailable from api/admin/channels.ts

backend/internal/handler/handler.go

@@ -33,7 +33,6 @@ type AdminHandlers struct {
 	Channel                *admin.ChannelHandler
 	ChannelMonitor         *admin.ChannelMonitorHandler
 	ChannelMonitorTemplate *admin.ChannelMonitorRequestTemplateHandler
-	AvailableChannel       *admin.AvailableChannelHandler
 	Payment                *admin.PaymentHandler
 }